This invention relates to an apparatus for destroying calculi in a living body, which comprises means for generating utrasonic waves outside the body and means for causing the waves to converge on a calculus within the body.
As is well known, calculi are formed in such organs of a living body as the kidney or gall-bladder. There are various ways of removing calculi from living organs, such as by conducting a surgical operation, melting them by chemical drugs, or destroying them by ultrasound. If the calculus is too large to be melted by chemicals, surgical operation or application of ultrasound is useful for removal of the calculus. Surgical operation, however, not only gives much pain to the patient but also requires careful aftercare of the patient. Use of ultrasound is advantageous since it suffers little from such difficulties.
As schematically shown in FIG. 2, one known apparatus for destroying calculi by ultrasound comprises a spherical shell 1 and a plurality of piezoelectric elements 2.sub.1 -2.sub.n arranged on the concave inner surface of the spherical shell at whose center the calculus to be destroyed is positioned. When the elements 2 are actuated simultaneously, they produce a beam of ultrasonic waves, and all the beams from the elements are focused on a point F set at the center of the spherical shell 1 to destroy the calculus positioned at the point F. A bag 3 filled with degassed liquid such as water and enclosing the piezoelectric elements is interposed between a patient K and the elements.
Another known device is schematically shown in FIG. 3 comprising a plurality of piezoelectric elements 2.sub.1 -2.sub.n arranged on the flat upper surface of a circular supporting plate 4. The elements are so controlled that the phase of the ultrasonic wave produced by each of the elements coincides with those of the waves produced by the other elements at a point F. A main controller MC produces a series of pulses, which are applied through delay circuits D.sub.1 -D.sub.n to high-voltage pulse generators G.sub.1 -G.sub.n, each of which actuates the corresponding one of the piezoelectric elements 2.sub.1 -2.sub.n to produce an ultrasonic beam, the delay circuits D.sub.1 -D.sub.n providing such different delay times that the ultrasonic beams produced by all the piezoelectric elements coincide in phase with each other at the point F so as to destroy a calculus positioned at the point. A bag 3 filled with degassed liquid such as water is interposed between the piezoelectric elements and a patient's body in which a calculus to be crushed is contained.
The arrangement of FIG. 2 has a disadvantage that the whole apparatus is bulky because of the spherical configuration of the shell 1 on which the ultrasonic transducers are arranged, and that it is difficult to make the apparatus of a compact size.
The arrangement of FIG. 3 has an advantage that the piezoelectric elements can be positioned near the body of a patient, thereby to make decrement of the ultrasonic waves caused by divergence less than in the arrangement of FIG. 2. The arrangement of FIG. 3, however, has the following problem. Suppose that a piezoelectric element in the form of a circular plate has a radius of b. As shown in FIG. 4, the ultrasonic beam produced by a piezoelectric element 2 diverges as it advances, and the half angle .alpha. of divergence to a zero pressure point is given by EQU sin .alpha.=0.6.lambda./b (1)
wherein .lambda. is the wavelength of the beam. The radius d of the sectional plane of the ultrasonic beam at the position of a calculus to be destroyed is given by EQU d=l tan .alpha. (2)
wherein l is the distance between the piezoelectric element and the position of the calculus, and .alpha. is the above-mentioned half angle of divergence to a zero pressure point of the beam.
If the radius b of the piezoelectric element is 1 cm and the frequency is 1 MHz, the angle .alpha. will be 5.degree., and if the distance l is 20 cm, the radius d of the section of the beam will be 1.75 cm. In order that the ultrasonic beams produced by the elements in FIG. 3 are focussed on the point F by controlling the phases of the beams, the sectional areas of the beams of all the elements must overlap one another. To make the radius d of the section of each beam at the point F, that is, the position of the calculus sufficiently large to enable phase control of the beams, the piezoelectric elements must have a smaller diameter, so that the number of piezoelectric elements to be provided increases with resulting increase in the number of high-voltage pulse generators and delay circuits to be provided and in the cost of manufacturing the apparatus.
Moreover, there is another disadvantage that due to the directivity of the piezoelectric element, an ultrasonic beam has an intensity distribution in the sectional area thereof, with the intensity becoming weak at the periphery, so that the efficiency at the focal point is reduced.